1. Field of the Invention
Pseudorabies (Aujeszky's disease) is caused by a herpesvirus belonging to the alphaherpesvirus subfamily. It is a contagious and sometimes fatal disease of swine. Infection during gestation can result in fetal death and abortion. It is estimated that annual losses to the swine industry due to pseudorabies is as high as 60 million dollars in the United States. This economic impact has resulted in a decision by the swine industry and regulatory officials to eradicate the pseudorabies virus (PRV). The virus also infects cattle, sheep, canines, and felines.
During the initial phase of the acute disease, PRV replicates in the upper respiratory tract. Virus can then disseminate by vascular, lymphoid and nervous tissues [D. P. Gustafson, In Diseases of Swine, ed. by A. D. Leman, et al., 6th edition, pp. 274-289, Iowa State University Press, Ames, IA]. Infectious virus and/or viral genome can be detected from lung, tonsil, brain stem, trigeminal ganglia and peripheral blood lymphocytes [F. Wang et al., J. Leukocyte Biol. 43: 256-264 (1988); G. Wittmann et al., Arch. Virol. 66: 227-240 (1980); H. J. Rhiza In Latent Herpes Virus Infections in Veterinary Medicine, ed. by G. Wittman et al., Martinus Nijhoff publishers, The Hague pp. 429-444 (1984); H. J. Rhiza et al., In Proc. 14th International Herpes Workshop, Nyborg, Denmark, p. 55 (1989)]. Clinical symptoms include intense pruritis, violent excitement, fits, paralysis, and eventually death. Upon cessation of clinical signs and recovery from infection, the virus is not eliminated from the animal and persists with the animal indefinitely. Sometimes, the infection is subclinical and goes unnoticed. The animal also becomes a carrier of pseudorabies. In either case, the virus exists in various cell types of the animal in a noninfectious form and is commonly known as a latent infection. The complete viral genome is present but fails to replicate fully to produce infectious virus. The latent virus can reactivate spontaneously or be induced to reactivate by exogenous stimuli, the carrier animal disseminates infectious virus to susceptible animals which may result in death of the animal or establishment of new PRV carriers. Thus, the latent virus is the source and reservoir of the disease and is regarded as an obstacle to the successful control and eradication of PRV.
This invention relates to a PRV deletion mutant vaccine characterized by a high degree of immunogenicity and yet a significant attenuation and reduced level of latency as compared to field strains of the virus.
2. Description of the Prior Art